In both the scientific and industrial sectors, conveniently obtaining accurate measurement of refractive index (RI) is of prime importance. In the context of the biological and chemical sciences, optical sensors which measure changes in refractive index are often used to detect elements in a medium by measuring changes in the refractive index of that medium. Consequently, detection of various kinds of elements such as bio-agents and pathogens contained in a medium are detected by RI measuring optical sensors through detection of changes in the medium's effective refractive index caused by the presence of those elements.
Photonic biosensors capable of measuring extremely small changes in the RI of biological/chemical samples have received considerable attention, and a number of optical platforms based on long-period gratings (LPGs), surface-plasmon polariton (SPP), and microstructured fibers have been investigated. Although these sensors are highly sensitive, exhibiting 1000-2000 nm/Refractive Index Units (RIU), often a precise determination of RI needs temperature isolation/calibration, since the RI of the waveguide regions change with temperature.
In the application of known fiber based optical sensors, typically, to measure the RI of a fluid sample, the optical sensor is immersed in the sample, a broadband light source coupled to one end of the sensor then transmits light through the sensor which is detected by a detector coupled to the other end of the sensor. The optical properties, i.e. the spectral transmittance, of the combination of the optical sensor and the sample will change based on the effective refractive index of the sample in the ambient region just outside the cladding of the fiber. Presence of an analyte in the sample being measured changes the RI of the sample, which affects the interaction of the sample with the evanescent field of the cladding mode which extends into the sample. Such changes in interaction change the “effective” index” of refraction through which the cladding mode propagates, which in turn shifts the resonance wavelength(s) and the resonance minima observed. Specific resonance minima of the spectral transmittance whose value of wavelength vary as a function of RI are typically used to determine the RI of samples and some optical sensor systems specifically rely upon the values of a plurality of these minima to provide a more accurate RI measurement.
The spectrum of the light transmitted through the optical sensor is not solely dependent on the effective refractive index of the sample but also on the temperature of the optical sensor. In particular the various resonance minima used to determine the RI of the sample can shift as a consequence of changing temperature. In order to ensure that the refractive index is measured as accurately as possible it is desirable to compensate for any change in the transmitted spectrum due to temperature. Obtaining accurate measurements independent of temperature can be approached utilizing one or more of a few broad techniques which include temperature isolation and regulation, optical sensor spectral transmittance temperature insensitivity, and mathematical compensation and calibration. Insofar as an optical sensor can be physically made to be temperature insensitive, the need for temperature isolation and regulation and/or mathematical compensation and calibrations is reduced as are measurement costs and time.
A number of different specific ways to facilitate temperature compensation have been suggested, such as combining fiber Bragg gratings (FBG) with LPGs in optical waveguides, cascaded LPGs in double cladding fiber, π/2 phase shifted dual resonance LPG (DRLPG), among others. All of these schemes, however, rely on measuring two parameters and calculating the desired measurement using a 2×2 matrix. This increases the complexity of the detection system. Other methods applicable to LPGs involve suitably chosen cladding and core materials with opposite thermo-optic coefficients and an overall coating of the sensor with a suitable composite material to compensate for the temperature-induced wavelength shifts. The requirement of custom-made fibers in the former increases the cost and fabrication time of the sensor while the extra coating in the latter reduces the RI sensitivity. Furthermore, often an attempt to increase RI sensitivity results in increased temperature sensitivity. It also is possible to fabricate and configure an optical sensor in such a way that it exhibits temperature insensitivity at a particular wavelength however this is not effective with approaches relying on multiple resonance minima.
It would be desirable to have an alternative to the above mentioned schemes, in particular one which would not require a 2×2 matrix, which is cost effective, and exhibits spectral temperature insensitivity over a wavelength range rather than at a single wavelength to enable use of multiple resonance minima and hence obtain measurements which simultaneously exhibit high RI sensitivity and high temperature insensitivity.